The present invention relates to portable apparatus for mixing and heating asphalt paving material and for recycling asphalt paving material which has been removed from a road surface.
Asphalt is widely used today as a road surfacing material and in other vehicular supporting applications such as parking lots and driveways. One of the primary reasons for the widespread use of asphalt is the relative ease with which it may be repaired particularly in the winter months when road deterioration is greatly accelerated. As is well recognized by any motorist, many potholes develop on asphalt roads during the winter due to the continuous freezing and thawing cycle. This causes the road surface to buckle or collapse thereby breaking up the asphalt layer which is then knocked loose by passing vehicles.
In general, asphalt paving material comprises an aggregate which is crushed and then mixed with asphalt or other suitable bituminous material which serves the function of the binder. When the binder is heated, it softens and becomes sticky, thereby permitting the aggregate to flow and forms a mixture which will adhere to the substrate.
One type of asphalt material often used for patching purposes is cold mix which is able to be laid in a cold state as opposed to hot mix which must be laid in a heated condition. It is common practice for the cold mix to be premixed at a central plant in advance and then stockpiled for use as a patching material at a later time.
In order to prevent the cold mix from becoming hard and unworkable in the stockpile especially during the winter months, the asphaltic cement is diluted with various petroleum based distillates such as kerosene or fuel oil. Although dilution is necessary to enable the cold mix to be worked, it is deleterious to the asphalt bonding quality and the various distillates must be evaporated before the patch has the same adhering and cementing qualities of the regular asphalt road surface. One problem which is quite prevalent is that of the patch mixture being tracked away from the required area by passing vehicles before enough of the diluting additives evaporate to allow the asphalt to again become a cementing agent capable of holding the aggregate particles together. Additionally, water in the repaired area frequently causes the asphalt to float out of the mix and leave uncemented aggregate behind. As more distillates are added to the asphaltic cement, the curing time increases and the water resistant qualities decrease thereby aggravating the above problems.
Additionally, a cold mix patch that does manage to cure sufficiently to stay in place during the cold weather months, often has heavy distillate additives trapped below the surface. Many of these patches tend to soften under the higher temperatures of the summer months thereby causing the material to become plastic and flow out of the repaired area.
In order to permit the asphaltic cement to regain its adhesive and bonding characteristics, the asphalt mixture is heated thereby driving off the diluting additives. This presents a problem of its own, however, since the boiling points for many of the additives are in the range of 550.degree. to 700.degree., which is sufficiently high to cause the asphalt to oxidize rapidly and even burn. In fact, the asphalt will flash at a temperature of approximately 425.degree. if there is a sufficient supply of oxygen. As will be described in greater detail hereinafter, the present invention prevents the oxidation or burning of the asphalt material within the drum by maintaining a relatively low oxygen level therein. Even if the oxygen content within the drum is controlled, the asphalt may flash when it exits from the drum because at this point it is exposed to an open flame and to a relatively high oxygen content atmosphere. For this reason, it is necessary that the temperature of the aggregate particles and asphaltic coating be controlled by accurately regulating the heating and the amount of time which the mixture remains within the drum. Avoiding direct contact between the material and the flame and rapidly discharging the material from the rear of the drum are also necessary to avoid oxidation.
In reclaiming and recycling asphalt pavement material, it is usually removed in relatively large chunks which must be substantially reduced in size either before or during the heating treatment. If the chunks are insufficiently broken up when they are discharged from the processing apparatus, the asphalt binder will not have softened and the material will not be sufficiently fluid and pliable to be spread. Also, the material will not have the adhesive and binding properties which are necessary to the formation of a satisfactory patch. On the other hand, if the larger chunks are heated through until the center has reached the proper temperature, the surface will most likely have been overheated so that it may catch fire either within the drum or as it is discharged onto the pavement.
Apparatus for heating and mixing asphalt patching material are generally known. One example of such apparatus is disclosed in U.S. Pat. No. 3,674,242 which comprises an inclined rotating drum having a plurality of longitudinally extending fins which serve to tumble the cold mix so that it is dropped through the stream of hot gas produced by an LP burner and simultaneously conveyed rearwardly. Since both the front and rear fins are radially oriented, the asphalt material is dropped through the hottest portion of the flame the entire length of the drum right up to the point where it is discharged. This often results in the material being overheated such that it oxidizes or flashes. If the flame intensity is reduced to prevent this situation, there is the danger that portions of the material will not be heated through sufficiently. U.S. Pat. Nos. 2,487,887 and 2,305,938 also relate to asphalt mixing and heating apparatus and although the mixing fins are angled in the direction of material flow, they extend inwardly from the drum surface in a radial direction. Furthermore, all of the fins are angled in this manner so that the material is advanced at the same rate regardless of its distance from the flame.
The prior art also fails to provide means whereby the larger chunks of asphalt material are broken up sufficiently before being discharged. As discussed earlier, this is an important factor in assuring that all the asphalt material is heated to the optimum temperature without oxidizing or burning the asphalt coating of the individual particles or chunks.